A vehicle seat device is known which includes a seat rail and a slide rail (for example, see Patent Literature 1). In the vehicle seat device disclosed in Patent Literature 1, the seat rail is slidable with respect to the slide rail fixed to a floor of a vehicle body and is locked in the slide rail by a slide lock mechanism. The lock mechanism includes a lock plate which can be engaged with the seat rail, a hinge pin which supports the lock plate rotatably, and a spring which biases the lock plate in a locking direction.